This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-010721, filed Jan. 19, 2000; and No. 2000-252564, filed Aug. 23, 2000, the entire contents of which are incorporated herein by reference.
This invention relates to an optical pickup apparatus for executing at least one of data recording and reproduction by applying a laser beam to a data recording medium.
A magneto-optical pickup apparatus is known as an apparatus capable of repeatedly recording and reproducing data. The most important demand for this apparatus is downsizing. To satisfy the demand, various means are being proposed. For example, Japanese Patent Application KOKAI Publication No. 10-143934 discloses an all-in-one type magneto-optical pickup apparatus as shown in FIG. 14.
This optical pickup apparatus comprises a silicon substrate 32, a hologram element 38 provided on the silicon substrate 32, a Wollaston prism 40 and a polarizing prism 39 provided on the hologram element 38. The hologram element 38 has a beam branching function. The Wollaston prism 40 and the polarizing prism 39 are formed integral as one body. The polarizing prism 39 has a polarization beam separating film 39a and an inclined surface 39b. The polarization beam separating film 39a has a transmittance of substantially 70% and a reflectance of substantially 30% for P-polarized light, and has a reflectance of substantially 100% for S-polarized light. The inclined surface 39b reflects light reflected by the polarization beam separating film 39a, thereby branching the light.
In the above-described optical pickup apparatus, a laser diode 33 emits a P-polarized light beam. The laser beam emitted from the laser diode 33 passes through the hologram element 38 and enters the polarizing prism 39. Part of the laser beam entering the prism 39 passes through the polarization beam separating film 39a, and is then converged by an objective 42 onto a magneto-optical disk 43. When the converged laser beam is reflected from the magneto-optical disk 43, the direction of polarization of the laser beam is rotated through substantially about 0.5xc2x0 by a magnetic signal recorded in the magneto-optical disk 43. As a result, the laser beam reflected from the magneto-optical disk 43 has a small amount of S-polarized light component that serves as a magneto-optical signal. This reflected light again enters the polarizing prism 39 via the objective 42.
Almost all the light having the S-polarized light component, which is contained in the light entering the polarizing prism 39, is reflected from the polarization beam separating film 39a of the prism 39, and then reflected from the inclined surface 39b of the prism 39. The thus-branched light enters the Wollaston prism 40. The light entering the Wollaston prism 40 branches into two polarized light beams. The polarizations of these two beams are perpendicular to each other. These two beams enter respective sections 36a and 36b of a photodiode 36 for data signal detection. Data detection is executed by detecting the difference between two signals output from the sections 36a and 36b. 
On the other hand, part of the light, which has the P-polarized light component and enters the polarizing prism 39, passes through the polarization beam separating film 39a and then enters the hologram element 38. At this time, xe2x80x9c+xe2x80x9d first-order diffracted light and xe2x80x9cxe2x88x92xe2x80x9d first-order diffracted light are generated. Thus, the light entering the hologram element 38 branches into two light beams. The xe2x80x9c+xe2x80x9d first-order diffracted light and xe2x80x9cxe2x88x92xe2x80x9d first-order diffracted light enter photodiodes 34 and 35 for error signal detection, respectively. At this time, the xe2x80x9c+xe2x80x9d first-order diffracted light is converged in a position between the hologram element 38 and the photodiode 34 by the lens effect of the hologram element 38. On the other hand, the xe2x80x9cxe2x88x92xe2x80x9d first-order diffracted light is converged in a position beyond the photodiode 35. Using this phenomenon, a focus error is detected by detecting the spot sizes of the light beams on the photodiodes 34 and 35. Further, an error in tracking is detected by detecting the positions of light spots on the photodiodes 34 and 35.
Since, in general, light emitted from a laser is divergent light, the magneto-optical pickup apparatus disclosed in Japanese Patent Application KOKAI Publication No. 10-143934 can use only the central part of a laser beam, and cannot use the peripheral part of the beam. Accordingly, the efficiency of use of light is low. This means that the disclosed apparatus cannot be used as a magneto-optical pickup apparatus that requires a high laser output, such as a rewritable magneto-optical pickup apparatus, or as a magneto-optical pickup apparatus in which the rotational speed of the disk is increased to increase the transfer rate.
Moreover, when using even the peripheral part of a laser beam, the disclosed apparatus requires a large polarizing prism that can receive a wide range of light. This makes it difficult to reduce the size of the apparatus.
In other words, the all-in-one type pickup apparatus disclosed in Japanese Patent Application KOKAI Publication No. 10-143934 can not satisfy the requirements of increasing the efficiency of use of light, that is, increasing the power of the light applied to the data recording medium as well as of downsizing the pickup apparatus.
It is the object of the invention to provide a compact optical pickup apparatus which has higher efficiency of use of light.
To attain the object, according to one aspect of the invention, there is provided an optical pickup apparatus for executing at least one of data recording and reproduction by applying a laser beam to a data recording medium, comprising: a semiconductor substrate having a main surface; a light source for emitting a laser beam in a direction substantially perpendicular to the main surface of the semiconductor substrate; an NA converter for receiving the laser beam emitted from the light source and for outputting the laser beam so that a divergent angle of the outputted laser beam smaller than a divergent angle of the received laser beam; an objective opposed to the data recording medium; a first beam branching unit for permitting one of transmission therethrough and reflection therefrom of the laser beam outputted from the NA converter to thereby guide the laser beam to the data recording medium via the objective so that the laser beam reflects from the data recording medium, the first beam branching unit also transmits therethrough and reflects therefrom a part of and another part of the laser beam reflecting from the data recording medium and then passing through the objective, respectively, thereby branching the laser beam in two directions; and a first light receiving element provided at the semiconductor substrate, for receiving the laser beam branched by the first beam branching unit.
According to the other aspect of the invention, there is provided an optical pickup apparatus for executing at least one of data recording and reproduction by applying a laser beam to a data recording medium, comprising: a semiconductor substrate having a main surface; a light source for emitting a laser beam in a direction substantially perpendicular to the main surface of the semiconductor substrate; a means for receiving the laser beam emitted from the light source, converting the received laser beam and for outputting the laser beam so that a divergent angle of the outputted laser beam smaller than a divergent angle of the received laser beam; an objective opposed to the data recording medium; a means for permitting one of transmission therethrough and reflection therefrom of the laser beam outputted from the divergent angle converting means to thereby guide the laser beam to the data recording medium via the objective so that the laser beam reflects from the data recording medium, and for transmitting therethrough and reflecting therefrom a part of and another part of the laser beam reflecting from the data recording medium and then passing through the objective, respectively, thereby branching the laser beam in two directions; and a means provided at the semiconductor substrate, for receiving the laser beam branched by the beam branching means.
The NA converter or the divergent angle converting means can pick up even a peripheral portion of a laser beam emitted from the light source. This enhances the efficiency of use of light. Further, since the NA converter or the divergent angle converting means reduces the divergent angle and the diameter of the laser beam emitted from the light source, a large optical component is not necessary even when picking up a peripheral portion of the laser beam emitted from the light source. Accordingly, a compact optical pickup apparatus can be produced.
Preferably, the NA converter has a numerical aperture of 0.15 or more at a side thereof close to the light source.
This NA converter can pick up a laser beam with a relatively large divergent angle emitted from the light source. Accordingly, the laser beam emitted from the light source can be used efficiently.
More preferably, the first beam branching unit transmits therethrough the laser beam outputted from the NA converter to thereby guide the laser beam to the data recording medium via the objective.
Between transmitted light and reflected light, the first beam branching unit shows different variations in skipping depending for incident angle. Specifically, in the first beam branching unit, the aberration of transmitted light is smaller than that of reflected light, and hence the spot size of the light transmitting the first beam branching unit and converged by the objective can be reduced to a value close to the diffraction limit. As a result, the spot size of the laser beam on the data recording medium can be reduced to substantially the same value as obtained in the case of no aberration. This enables accurate and high-density recording and/or reproduction of data.
Also preferably, the optical pickup apparatus further comprises a second beam branching unit provided between the light source and the first beam branching unit, for transmitting therethrough, in one direction, a part of the laser beam reflecting from the data recording medium and passing through the objective, and branching therefrom, in another direction, another part of the laser beam reflecting from the data recording medium and passing through the objective; and a second light receiving element for receiving the laser beam branched by the second beam branching unit.
The second beam branching unit and the second light receiving element constitute an error signal detection system, which enables the production of a more compact and high-density optical pickup apparatus.
Yet preferably, the second beam branching unit is provided between the light source and the NA converter.
Since no optical component is provided between the second beam branching unit and the light source, the optical positions of the light source and the second light receiving element are secured with high accuracy. Accordingly, the data recording medium and the second light receiving element can be easily made to have substantially a conjugate positional relationship. This being so, error detection such as detection of the size or the position of a light spot on the second light receiving element can be executed reliably at all times.
Further preferably, the NA converter is provided between the second beam branching unit and the light source.
Since the distance between the NA converter and the light source can be reduced, a laser beam emitted from the light source enters the NA converter before it diverges. Accordingly, the beam diameter of the laser beam passing through the NA converter is reduced, which means that the optical pickup apparatus can be made more compact.
Preferably, the NA converter is provided between the second beam branching unit and the second light receiving element, so that the laser beam branched by the second beam branching unit passes through the NA converter and reaches the second light receiving element.
Since both the beam branched by the second beam branching unit and reaching the second light receiving element, and the beam emitted from light source and reaching the second beam branching unit pass through the NA converter, it is easy to adjust the optical distance of the former light and the optical distance of the latter simultaneously. As a result, the data recording medium and the second light receiving element can be easily made to have substantially a conjugate positional relationship. This being so, error detection such as detection of the size or the position of a light spot on the second light receiving element can be executed reliably at all times.
Preferably, the NA converter is provided between the second beam branching unit and the second light receiving element, so that the laser beam branched by the second beam branching unit reaches the second light receiving element without passing through the NA converter.
Light branched by the second beam branching unit and reaching the second light receiving element does not pass through the NA converter, while light emitted from the light source and reaching the second beam branching unit passes through the NA converter. Accordingly, the distance between the light source and the second beam branching unit can be made shorter than the distance between the second light receiving element and the second beam branching unit. This increases the number of manners that can be used to mount the semiconductor laser.
Preferably, the second light receiving element is provided at the semiconductor substrate.
Since the first and second light receiving elements are formed in surface portions of a single semiconductor substrate, the number of their component parts is reduced, and hence the cost of the light receiving elements can be reduced.
Further preferably, the second beam branching unit includes a hologram element.
Since the angle between light beams branched by the hologram element is smaller than the angle between light beams branched using reflection of, for example, a prism, the optical pickup apparatus can be made more compact.
Yet preferably, at least, the semiconductor substrate, the light source, the first beam branching unit, the second beam branching unit and the NA converter are integrated as one body.
Since these component parts are formed integral as one body, the optical pickup apparatus can be made more compact.
Also preferably, the NA converter is integral with at least one of the first beam branching unit and the second beam branching unit.
Forming the NA converter integral with an optical branching element enables reduction of the number of the component parts of the optical pickup apparatus, and hence reduction of its cost.
It is desirable that the optical pickup apparatus further comprises a collimator lens provided between the first beam branching unit and the objective, so that the collimator lens converts the laser beam outputted from first beam branching unit into substantially a parallel laser beam.
Since this optical system is an infinite optical system in which parallel light is created using the collimator lens and converged using the objective, there is no change in the output power of the objective even when the objective is shifted along the optical axis for the purpose of focusing control. Therefore, high output power can be obtained reliably. Moreover, since in this optical system, a separating optical system, in which only the objective is incorporated in a movable section, can be achieved, power saving or high-speed operation can be realized.
Preferably, the collimator lens corrects spherical aberration that occurs in the NA converter, the first beam branching unit and the second beam branching unit.
Correction of aberration using the collimator lens enables reduction of the spot size of light converged on the data recording medium by the objective to a value close to the diffraction limit objective. As a result, high-density recording and/or reproduction and increase of the capacity of a single sheet of data recoding medium can be realized.
It is also preferable that the optical pickup apparatus further comprises a polarization beam separating unit for separating, into a plurality of polarized laser beams, the laser beam reflecting from the data recording medium, passing through the objective and branched by the first beam branching unit, the polarized laser beams being received by the first light receiving element.
Since the polarization beam separating unit and the first light receiving element can constitute a system for detecting a magneto-optical signal, a more compact optical pickup apparatus of a higher density can be made.
Preferably, the polarized laser beams are focused on a surface of the first light receiving element.
Since the beams are focused on the first light receiving element, the area of this element can be reduced. As a result, the frequency bandwidth of the first light receiving element can be widened, and hence the optical pickup apparatus can be operated at high speed. It is possible that there are advantages similar to that occurs when the beams are focused on the first light receiving element.
Further, the first beam branching unit may be formed of a polarization beam splitter having a transmittance and a reflectance that vary in accordance with a polarization of light entering the polarization beam splitter.
Since the polarization beam splitter enables efficient transmission/reflection of light using polarization characteristics, the efficiency of use of light and the quality of a signal can be enhanced.
Furthermore, the NA converter may be formed of a convex lens having two opposite surfaces, one of the two opposite surfaces including a convex surface, and the other including a flat surface.
Forming only one surface of the NA converting lens as a convex surface can reduce the cost of the lens. Moreover, since the other surface of the lens is a flat surface, mounting of the lens in the apparatus or attachment of the lens to an optical element for branching light can be executed easily.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.